The invention relates to a process for producing oxygen-free or low-oxygen shaped articles or bodies of silicon carbide, in particular SiC fibers, using chemically reactive polysilanes or polycarbosilanes as precursors.
The production of SiC fibers using polycarbosilanes is known. Thus, for example, by pyrolysis of polydimethylsilanes, methyl-containing polycarbosilanes are initially produced, which are spun into fibers and are stabilized by oxidation, whereby a Si-O containing support layer is formed. The treatment with oxygen causes the resulting pyrolyzed SiC fiber to have an SiO.sub.2 content of 20% However, when the fiber is subjected to thermal stress, this high proportion of Si-O bonds leads to deterioration of its mechanical properties and thus to destruction of the fiber.
It is generally true that the stability of an SiC fiber at temperatures&gt;1000.degree. C. decreases dramatically as the oxygen content increases.
In accordance with U.S. Pat. Nos. 3,052,430 and 4,100,233, polysilanes are thermally transformed to polycarbosilanes, and these are then spun into green fibers and pyrolyzed under vacuum for 2 to 48 hours at temperatures of from 350.degree. to 400 .degree. C. Stabilization of the fiber form is likewise effected by oxidation at 50.degree. to 400 .degree. C.
U.S. Pat. Nos. 4,220,600 and 4,283,376 describe processes for producing SiC fibers which utilize polycarbosilanes containing SiO in their molecular skeletons as starting materials. Polycarbosilanes of this type are obtained by pyrolysis of polysilanes in the presence of 0.01 to 15 wt-% added polyborosiloxane. Stabilization of the spun fibers is likewise effected by heating in an oxidizing atmosphere at temperatures between 50.degree. and 400 .degree. C., whereby Si-O bonds are formed.
Stabilization by exposure to gamma-radiation or electron-radiation in an oxidizing or non-oxidizing atmosphere is likewise described.
According to U.S. Pat. No. 4,737,552 a mixture of polycarbosilane, a catalyst for hydrosilation, and an unsaturated compound are spun into a green fiber which is rendered infusible by thermal treatment at temperatures of 150.degree. to 400 .degree. C. in an inert atmosphere or in a vacuum and subsequently pyrolyzed.
U.S. Pat. No. 5,171,722 starts from a solution of a polycarbosilane, a polyvinylsilazane, and a catalyst, which preferably generates H.sub.2 or CH.sub.3 radicals. Green fibers spun from the solution are heated in the temperature range from 25.degree. to 200 .degree. C. for a period of time from 0.5 to 24 hours and stabilized. The subsequent pyrolysis takes place at 600.degree. to 1000 .degree. C. under an inert atmosphere or in a vacuum.
Published European Patent Application No. EP 496,574 describes the production of SiC fibers with a low oxygen content. In this case the stabilization of the fiber backbone takes place during thermal treatment of a green polycarbosilane fiber in an atmosphere of hydrocarbons, halogenated hydrocarbons, or unsaturated hydrocarbons.
All of the aforementioned processes have in common the need to stabilize the form of the spun green fiber, that is to make it infusible. The method used on an industrial scale is oxidative protection of the green fibers. However, due to the effects of oxygen, the thermal resistance of the SiC fibers is sharply decreased, so that the fibers are not suitable for use at high temperatures.
The possibility of stabilizing the fibers by exposure to radiation is very cost intensive, and it is therefore utilized only for limited industrial fields of application or at laboratory scale.